Notes Payable - Recognized Amounts in Interest Expense in the Condensed Consolidated Statements of Income Related to the 4.00% Debentures (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
Jul. 31, 2014	Jul. 31, 2013	Jul. 31, 2014	Jul. 31, 2013
Debt Instrument
Interest expense at the contractual interest rate	$ 4,807	$ 4,897	$ 9,392	$ 9,682
4.00% Debentures due 2031
Debt Instrument
Interest expense at the contractual interest rate	2,530	2,530	5,060	5,060
Amortization of debt discount	$ 1,521	$ 1,416	$ 3,015	$ 2,807

Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 1A

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

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-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

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